The present invention relates to an apparatus for supplying foil material, for example for producing foil bags, wherein foil material is supplied by a supply roll and is withdrawn by a withdrawal means, and to a corresponding method.
In a process in which foil material is supplied by a supply roll in one piece and is removed by a withdrawal means to be further processed, wherein the withdrawal means has a predetermined rate or speed adapted to the processing speed of subsequent processing stations, the foil material in front of the withdrawal means must be under tension. The supply roll is rotatably supported and is rotated by the withdrawn foil.
Such a foil supply means is e.g. needed for producing foil bags. Such foil bags comprise, for example, two rectangular side foils which in the filled state are respectively sealed to each other at the side edges. A bottom or stand-up foil is optionally sealed in between the four edges and is folded open such that there is room for filling material in the foil bag. Such a filling material may e.g. be a beverage.
In known supply means the tension in front of the withdrawal means is provided by a co-called tensor means. The foil is guided over a plurality of stationary and movable rollers which are alternately arranged. The movable rollers are biased by a spring force which becomes the greater the smaller the distance becomes between the movable and the stationary deflection rollers. The foil is thus tensioned all the time.
In such a prior-art supply means the withdrawal force which is needed for withdrawing a certain amount of foil material from the supply roll is not constant, as the diameter of the supply roll is decreasing the more the foil material is unwound, and the torque to be applied thus changes accordingly, as a result of which the tension of the foil changes. On the other hand, the tension of the foil is not constant since the tensor means does not exert a constant force on the foil material when the distance between the movable and the stationary deflection rollers changes since the spring force which acts on the movable rolls is in proportion to the longitudinal extension of the spring.
However, in view of the high precision needed for producing foil bags, with very high speeds being realized in an automatic supply means, it is very important that the foil material be supplied in an exact manner. Therefore, the tension in front of the withdrawal means should be as constant as possible.
It is therefore the object of the present invention to provide an apparatus and a method for supplying foil, with the help of which a substantially constant tension is achieved in front of the withdrawal means.
In the supply method according to the invention, the foil material is supplied by a motor-driven supply roll, subsequently guided around at least one stationary deflection roller and at least one sensor roller whose distance from the at least one deflection roller is variable, and is finally withdrawn by a withdrawal means, the speed of the drive of the supply roll being increased when the distance of the sensor roller from the deflection roller is smaller than a first predetermined distance, and the speed of the drive being reduced when the distance of the sensor roller from the deflection roller exceeds a second predetermined distance.
To this end, the supply apparatus of the invention is provided with a motor-driven supply roll having a supply capacity, with a withdrawal means having a withdrawal capacity, and with a consumption sensor by which the drive of the supply roll is controlled in response to the consumption of foil material, and which is arranged between the supply roll and the withdrawal means, wherein the consumption sensor comprises at least one stationary deflection roller, at least one sensor roller which is supported and arranged such that it changes its distance from the deflection roller in accordance with the difference between supply capacity and withdrawal capacity, and a sensor means which senses the change in distance and which increases the speed of the drive of the supply roll when the distance is smaller than a first predetermined distance and which decreases the speed of the drive when a second predetermined distance is exceeded.
Thanks to the method of the invention and the apparatus of the invention, the motor drive of the supply roll achieves the effect that no additional force is needed for the withdrawal operation. As a result, a varying withdrawal force is not observed either when the actual diameter of the supply roll is changed by the measure of unwinding the foil material. However, the supply roll is not driven in a uniform manner. When the rotation of the supply roll is slowed down, the distance between the at least one sensor roller and the at least one deflection roller is reduced by the withdrawal means. The force which is here exerted by the sensor roller on the foil material is constant, so that the tension applied to the foil material in front of the withdrawal means remains constant during said process. It is only when the sensor roller has a distance which is smaller than a first predetermined distance from the deflection roller that the drive of the supply roll is accelerated by a sensor means. The distance of the at least one sensor roller from the at least one stationary deflection roller is then increased again accordingly. However, during such a process the tension in front of the withdrawal means is also determined by the force which is exerted by the sensor roller on the foil material, and is thus constant since no spring elements are provided for. Hence, a constant tension of the foil material is ensured during the entire operation with the method according to the invention and with the apparatus according to the invention.
A deceleration or acceleration of the supply roll without the roll being entirely stopped may be advantageous in cases where very large and thus heavy supply rolls are used. The forces exerted on the drive of the supply roll are thereby reduced. In another development the supply roll is possibly completely switched off when the second predetermined distance is exceeded, and is possibly switched on again when the distance is smaller than the first predetermined distance. Such a design may be of advantage in cases where the position of the sensor roller should be corrected as fast as possible or where the control unit should have a design which is as simple as possible.
The sensor roller may be freely suspended e.g. held by the foil. Advantageously, however, there is provided a guide means along which the at least one sensor roller moves while the distance is changed so as to ensure a reliable guidance of the at least one sensor roller without the occurrence of any malfunctions caused by a displacement.
A particularly frictionless sliding movement of the sensor roller within the guide means is accomplished when said means is oriented in a substantially vertical direction.
The at least one sensor roller can maintain the constant tension within the foil material by its own weight. Depending on the demands, it may additionally be loaded by further weights to adjust a predetermined tension.
According to a preferred development, the sensor means comprises a first sensor which outputs a signal for accelerating the drive when the distance of the at least one sensor roller from the at least one deflection roller is smaller than a first predetermined distance. In a further development, the sensor means comprises a second sensor which outputs a signal for slowing down the drive when the distance of the at least one sensor roller from the at least one deflection roller exceeds the second predetermined distance. With such sensors, the motor control can be realized in a very simple way.
In an advantageous further development, the apparatus according to the invention comprises a first safety sensor which produces an error signal when the at least one sensor roller has a distance smaller than the minimum distance from the at least one deflection roller. In a further development the sensor means comprises a second safety sensor which produces an error signal when the at least one sensor roller exceeds a maximum distance from the at least one deflection roller. Should a malfunction occur or should the supply roll be completely unwound, an error signal is produced in these further developments, for instance, in order to produce an alarm signal for warning the operating personnel.
The sensors or safety sensors may e.g. be formed by mechanical switches which are activated by the moving sensor roller. By contrast, a particularly simple and reliable configuration provides for optical sensor elements, such as light barriers. The movement of the sensor roller is not influenced by the contactless operation of such optical sensors, whereby the constant tension of the foil material is further maintained.
In another embodiment proximity switches are provided as sensors or safety sensors which ensure a simple and reliable structure which, in addition, is less prone to soiling.
In an advantageous development of the method according to the invention, the withdrawal means and the operational components for the further processing of the foil are switched off when the distance is smaller than a minimum distance. In case of a malfunction, or in cases where the supply roll is not entirely unwound, the feeding operation will thus be interrupted until a normal operation is possible again. According to a further development, the motor of the supply roll, the withdrawal means and the operational components for further processing the foil are switched off when a maximum distance of the at least one sensor roller from the at least one deflection roller is exceeded. When a maximum distance is exceeded, there is obviously a malfunction in the discharge of the foil material, so that the motor of the supply roll must be switched off in addition to interrupt the further supply.
For implementing such developments of the method according to the invention, an advantageous development of the apparatus according to the invention provides that the error signal which is produced by the respective safety sensors should be used at least for switching off the withdrawal device and those operational components that are used for the further processing of the foil supplied.
Depending on the respective demands, the withdrawal means is operated continuously. Even in the case of an intermittent operation of the withdrawal means, the apparatus of the invention and the method of the invention can advantageously be used since the apparatus according to the invention and the method according to the invention also ensure in such a case that the tension of the foil material in front of the withdrawal means remains constant.
If a plurality of individual foil webs are required for the further processing of the foil, the apparatus according to the invention can be used in a plurality of parallel supply means repeatedly. For instance, two foil webs can be used for supplying foil material to form the respective two side foils of a foil bag. Advantageously, the motors of the supply rolls are activated independently of each other by a joint control unit.